In conventional shot peening, small balls are fired against the surface of a metal target to create plastic deformation and corresponding residual compressive stress in the target workpiece. The residual compressive stress improves the useful fatigue life of the workpiece when it is used in a high stress application. In an exemplary application, conventional shot peening has been used in the manufacture of blades for a gas turbine.
It is known to use laser shock peening in place of conventional shot peening. Here, a cylindrical laser is operated in a pulse mode for directing laser pulses against a workpiece surface. Typically, the workpiece surface has a light-absorbing ablative coating (e.g., black paint) which is covered by a thin layer of water. The laser pulse vaporizes the coating in a small explosion which is confined by the water to develop an instantaneous pressure pulse. The pressure pulse causes plastic deformation and corresponding residual compressive stress in the target workpiece.
Preferably, the laser pulse is generated by a Q-switched oscillator having, in optical alignment, a totally-reflecting mirror, a quarter wave plate, a Pockels cell, a polarizer, an optically-pumped laser rod, and a partially-reflecting mirror. By switching the biasing voltage to the Pockels cell, the laser pulse is removed from the oscillator through the partially-reflecting mirror, passed through an optical amplifier, and directed against a first circular area on the target surface. Additional laser pulses are directed to adjoining, and partially overlapping, circular areas to adequately cover the target surface.
What is needed is a more efficient method for laser shock peening a target surface.
Conventional lasers include slab lasers wherein the lasing medium is a slab having a shape of a generally rectangular parallelepiped. Such lasers produce a laser beam having a cross section taken perpendicular to the laser beam, wherein the cross section has a generally rectangular shape. Conventional slab lasers include those which operate in a pulse mode.
It is known to use a conventional optical amplifier in which a laser beam is passed several times by reflecting mirrors through the amplifier. For a slab laser, the laser beam has a rectangular cross section taken perpendicular to the laser beam, and the laser beam typically is directed to zigzag through the amplifier by total internal reflection to avoid thermal focusing caused by uneven temperature distribution of the slab material, as is known to those skilled in the art. However, employing the total internal reflection technique requires optically polished slab surfaces. Slab materials are known which do not have thermal focusing problems, and the laser beam may be passed straight through optical amplifiers having such slab materials without the need for optically polished slab surfaces. However, the amplified laser beam is somewhat inhomogeneous due to the amplifier's nonhomogeneous gain distribution. It is noted that such laser beam inhomogeneities are self-corrected by the internal reflections of the zigzag technique.
What is needed is a reflective laser beam homogenizer which can be used for diverse purposes such as, but not limited to, homogenizing an amplified laser beam which has a rectangular cross section taken perpendicular to the laser beam and which has passed straight through an optical amplifier.